In recent years, from the viewpoint of environmental load reduction, there has been a need for environmentally friendly and clean liquid fuels with a low sulfur content and aromatic hydrocarbon content. Therefore, in the oil industry, as a production method of clean fuels, the Fischer-Tropsch synthesis method (hereunder also referred to as “FT synthesis method”) using carbon monoxide and hydrogen as feedstocks is being investigated. The FT synthesis method has very high expectations since it can manufacture a liquid fuel base stock rich in paraffin content and not containing a sulfur content, such as a diesel fuel base stock. For example, environmentally friendly fuel oils are also proposed in Patent Document 1.
Synthesis oils obtained by the FT synthesis method (hereunder also referred to as “FT synthesis oil”) have a large n-paraffin component, and even if a diesel fuel base stock is obtained by fractionally distilling this FT synthesis oil, there is a concern that as is, the low-temperature performance of the diesel fuel base stock is insufficient.
Furthermore, a substantial amount of a wax fraction, which is a heavier component than the diesel fuel base stock, is simultaneously produced along with the diesel fuel base stock. Therefore, if a middle distillate which is lighter than the wax fraction can be produced by hydrocracking the wax fraction, it will lead to increased production of the diesel fuel base stock.
Therefore, a production method for a diesel fuel base stock that, in addition to producing a diesel fuel base stock by mixing a hydroisomerized first middle distillate and a middle distillate corresponding portion (cracked wax fraction) which is lighter than the wax fraction produced by hydrocracking the wax fraction, selectively decreases the n-paraffins in the heavy portion of the diesel fuel base stock obtained at that time, is proposed (Patent Document 2). According to this production method, the low-temperature properties of the diesel fuel base stock itself can be improved.